PBL 3853
Expression for DC-characteristics.
from figure16, (empirical).
V
F
=
V
L
−
(
(
V
L
−
1.3
−
V
DC
)
0.07
⋅10
−3
+
0.6⋅10
−3
)
(
R
1
+
R
2
)
....................................................................................8
V
X
=
I
L
=
(
V
F
−
2.0
)
/ 45
+
0.14
0.18
.......................................................................................................................................9
5.8
(
V
X
−
1
)
R
12
0.65
...........................................................................................................................10
1+
+
R
13
R
5
R
13
If the resistors R
A
and R
B
are connected in the circuit as in figure 16, the V
X
will have somewhat more complex expression:
V
X
=
(
V
F
−
2.0
)
/ 45⋅10
3
+
0.14
⋅10
−3
+
(
Z V
L
)
/
(
2.5⋅10
3
R
A
)
................................................................................11
0.18⋅10
−3
+
1/ 2.5⋅10
3
−
Z
2.5⋅10
3
(
)
2
1
1
1
0.65
=
+
+
Z
R
A
R
B
2.5⋅10
3
If the resistors R
A
and R
B
are set to infinity in this expression it will take the same form as above.
NOTE
1. These expressions are
built on the asumption that the
β
of the transistors is infinite
and that the influence of
capacitors except C
2
can be
neglected.
2. These expressions are built
on an ideal situation and are
only for help to understand
Telephone
set side
Line side
Active impedance
section
Telephone
I
Load
I
1
R12
R13
Line
I
L
γ
-
V
Z
p
Z
Line
V
L
δ
α
+
+
-
V
R5
a
PBL 3853
T
x
2
b
15
Passive impedance
section
R
x 16
Z
2
R
6
R
8
C
R
7
R
11
Z
1
Figure 16.
R
5
C
6
Z
bal
R
9
R
10
MUTE and Cut-off Function
The receiver mute (pin 17) is a cut-off function. The normal
receiver amplifier is cut-off but a second amplifier is activated
to let the DTMF or the payphone signal injected to pin 17 get
through to the earphone. The input level for cut-off:
V cut-off = Rx output (VDC/2) + 2x V diode
≈
2.2 V + 1.3 V
≈
3.5 V
The signal for DTMF confidence tone in the earphone must
be injected to pin 17 at correct DC level or via a capacitor.
The mute signal is taken from pin 18 via a resistor in series
with at diode. The diode is needed in order not to disturb the
DC-level at pin 17 in not muted condition.
The transmit mute (pin 10) can be taken from the same point
(pin 18) with a series resistor (see figure 10).
Figure 14.
I
L
V+L
R
11
V
TO
R
6
R
7
R
10
R
5
Zbal
RI
Z in
R
19
Z
Line
Figure 15.
9
ERICSSON [ ERICSSON ]
